1. Field of Invention
The present invention relates to a device which separates into heavy and light liquids and solids a fluid consisting of heavy and light liquids and solid objects.
2. Description of the Prior Art
Heretofore, means, such as a centrifugal separator, for separating a fluid consisting of a mixture of heavy and light liquids and solids having a larger specific gravity than the heavy liquid into three phases of heavy and light liquids and solids have been utilized. Conventionally utilized three-phase separation means include, for example, means for separating a liquid into heavy and light liquids with a disc-type decanter after separating solids from the liquid with a common centrifugal separator or filter.
Three-phase separation utilizing, for instance, the screw-type decanter shown in FIG. 3 has been tried. As shown in FIG. 3, a screw-type decanter 20 consists of a bowl 21 comprising a cylindrical section 21a and a conical section 21b connected thereto, and a screw which is mounted coaxially in the bowl 21 with a small clearance between its pheripheral portion and the inner wall of the bowl, wherein, when the bowl 21 is rotated at a high speed, a fluid of a mixture of solids and a liquid poured (arrow A') into the bowl 21 is separated into solids indicated by arrow B' and light and heavy liquids indicated by arows C' and D', respectively. The taper angle .theta. of the conical section needs to be made small in order to effectively expel the solids. Generally the overall length of the decanter 20 cannot be made unduly long considering, the installation space and other factors. The length L.sub.1 of the conical section 21b cannot be made long in order to keep the length L.sub.2 of cylindrical section 21a long because of the limitation of the overall length L.sub.0. Therefore this short length L.sub.1 of the conical section as well as its small taper angle .theta. inevitably render the depth D of the fluid in the bowl 21 shallow.
In the invention disclosed in Japanese Patent Laid-open No. 50-54161 by the inventor, a screw-type decanter, such as shown in FIG. 3, has two-staged tapering angles .theta..sub.1 and .theta..sub.2 of the conical section 21b as shown in FIG. 4, wherein the tapering angle .theta..sub.1 of the portion adjacent to the cylindrical section 21a is made larger, and the tapering angle .theta..sub.2 of the next portion is made smaller. This design permits the depth D of the fluid (FIG. 3) to be relatively large. This type of decnater only permits the separation of two phases of liquid and solids, but not the separation of light and heavy liquids.
The means described above, in which a liquid is separated into heavy and light liquids with a disc-type decanter after solids are separated from a fluid with a centrifuge or filter, requires two sets of centrifuges, requiring two treatments of high energy consumption, resulting in high costs for installation and operation.
A screw-type decanter, such as shown in FIG. 3, cannot fully separate two phases of liquids (heavy and light liquids) because of the shallow depth D of the liquid in the bowl, so that it is common practice to repeat the separation treatment with a disc-type decanter, for example, to separate the liquids.
An example of test result data utilizing the duplicated separation treatments described above with a two-phase separation decanter and disc-type decanter (separation plate-type centrifuge) is shown in the block diagram in FIG. 5. As shown in the data in FIG. 5, which are test results in a fish meal plant, oil in the light liquid was accompanied by water of 2.5% by volume and solids of 2.0% by volume, requiring polishing with a separation plate-type centrifuge (disc-type decanter) as an after-treatment. Thus, three-phase separation with a single treatment, in which a reduction in construction cost due to a reduced number of machines and a reduction in energy cost due to reduced operations can be attained, was not accomplished.
The reasons for this is as follows. In the screw-type decanter described above in reference to FIG. 3, the depth D of the fluid in the bowl 21 is inevitably shallow, so that the separation of solids from the liquid becomes insufficient when the specific gravities of the solids and liquid in a suspension fluid to be treated do not show much difference.
Furthermore, if the decanter of this type is used for three-phase separation, it is difficult to set an interface between the heavy and light liquids due to the insufficient, shallow depth D of the liquid and turbulence from the fluid flow, the turbulence also stirring up the solids, thereby rendering satisfactory separation impossible.
A screw-type decanter having two-staged tapering angles in the conical section as described in reference to FIG. 4 is conventionally used to separate two phases, i.e., solids and liquids, but is not constructed to properly separate three phases, i.e., heavy and light liquids and solids.